Despite ongoing concerns about security, demand for air travel is forecast to continue at an extraordinary rate in both mature and developing markets. In the USA, the NGATS program forecasts that passenger numbers will increase by up to 140% over the next 20 years with aircraft movements increasing up to three-fold, depending on the mix of small and larger aircraft. See, e.g., Next Generation Air Transport System Integrated Plan, JPDO, December 2004, incorporated herein by reference. In Europe, the SESAR Consortium predicts similar challenges, with the number of flights predicted to increase by 150% over the same period. See, e.g., SESAR Definition Phase Deliverable for Air Transport Framework—The Current Situation, SESAR Consortium 2006, incorporated herein by reference. In developing markets such as China, Asia-Pacific and South America the growth is expected to be even greater. See, e.g., Boeing Current Market Outlook 2006, incorporated herein by reference.
From an ATM perspective, the result will be around twice as many commercial aircraft travelling a more complex network of point-to-point and hub-and-spoke routes to an increasing number of airports. In turn, this will require reduced separation and flexible route planning, which will place significant pressure on improved performance from ATC systems and surveillance technologies.
There is general agreement that Automatic Dependent Surveillance—Broadcast (ADS-B) will play a significant role at the core of future civil aviation surveillance infrastructure and, following some years of pilot programs (including CAPSTONE in USA, the Bundaberg Trials in Australia, and CASCADE/CRISTAL in Europe) and discussions about standards and technologies, major ADS-B deployment programs are now in progress, including the Australian Upper Airspace Program and the FAA's NAS-Wide ADS-B Program.
ADS-B uses new on-board avionics subsystems which incorporate GNSS positioning systems (e.g., GPS or alternatives such as Europe's Galileo), an interface to flight management systems, and a transponder to broadcast aircraft position and supplementary information on a regular basis. This approach offers a number of benefits, especially when compared to traditional radar alternatives:                The ground infrastructure required to determine aircraft position is relatively cheap, consisting of radio receivers able to detect and decode the messages within line-of-sight of the transmitting aircraft and up to 250 nm distant from the aircraft.        The resulting data is generally extremely accurate (potentially within tens of meters), of high integrity, and with an update rate far exceeding that obtained from radar.        ADS-B architecture is two-way, which allows aircraft to receive position and other information directly from other aircraft or from ground based (TIS-B and FIS-B) infrastructure, to provide rich cockpit information and enable new cockpit-based applications.        
These benefits present an overwhelmingly compelling case for ADS-B deployment, however there are a number of challenges, which must be addressed for ADS-B to be adopted as a primary or sole surveillance solution in order that the benefits can be completely realized.
Firstly, ADS-B requires new equipment on-board each aircraft and, while incremental costs of ADS-B equipage for new aircraft is small, the costs to retrofit existing aircraft, including certification costs and the opportunity costs of the associated operational downtime, are significant. As a result, even with new rules and mandates being introduced, it will be many years until equipage levels are such that ADS-B can be used as a platform for consistent and universal aircraft separation. See, e.g., Paper to RTCA Spring Forum—Mitre Corporation, May 2005, incorporated herein by reference. The FAA ADS-B program assumes that, even with rulemaking support, it will be 2020 until all large commercial aircraft are equipped.
Secondly, the transition from radar to ADS-B will need to address issues of data integrity and validation before ANSP's can undertake safety-critical separation services using position information derived no longer from their own radar infrastructure but from information provided directly from the aircraft avionics. It seems likely that, even if the safety case supports ADS-B-only surveillance, issues of governance and responsibility will require ANSP's to establish an independent means of backup and validation for ADS-B and the associated business case will be significantly impacted if this backup system must rely only on ongoing use of radar infrastructure. Encrypting ADS-B has been proposed as a means to validate ADS-B, but this technique does not support backup surveillance. See, e.g., Digital Avionics Systems Conference (DASC)—Sensis, October 2006, incorporated herein by reference.
Thirdly, surveillance based on the broadcast of self-reported aircraft position raises security issues, both in terms of the ease with which aircraft can be tracked from the ground by almost anyone, using low-cost and readily available ADS-B decoding units, and also by the potential for an aircraft to knowingly mislead a surveillance system by spoofing its position information and appearing to be in a position other than the position at which it is actually located. In May 2006, the potential vulnerabilities to spoofing were described in a letter from the Australian Civil Aviation Authority's former Chairman to the Australian Government's Minister for Transport and Regional Services, which highlighted the potential for malicious or capricious actions, stating that “any electronics boffin, using a second-hand or ‘borrowed’ transponder from a small GA aircraft connected to a $5 data lead, a $5 aerial and a laptop computer, can create ten, twenty or even fifty false aircraft on an air traffic controller's screen.” See: Open letter from Mr Dick Smith to Australian Minister for Transport and Regional Services—May 2006 6, incorporated herein by reference
Finally, the introduction of ADS-B surveillance will require the finalization and global adoption of a significant new body of associated standards for both aircraft and ground domains. See, e.g., Reference Safety, Performance and Interoperability Requirements Document for ADS-B-NRA Application—ED 126 Draft. EUROCAE, August 2006, incorporated herein by reference.
It can be argued that multilateration techniques can be purposefully, economically, and effectively integrated into an ADS-B surveillance infrastructure to mitigate these issues and to enable a faster, more comprehensive, and more cost-effective ADS-B implementation.
In doing so, the term “Extended ADS” (ADS-X) is sometimes used to describe this integrated approach, as it avoids the traditional and, unhelpful tendency to compare ADS-B and multilateration techniques and the implication that we are somehow choosing between the two technologies.
Multilateration systems use triangulation techniques to determine the source of transponder emissions by analyzing the time difference of arrival (TDOA) of those signals at a network of receiving ground stations with three or four stations required to receive each signal in order for the central processor to determine a triangulation outcome.
These systems are well-proven around the world in Advanced Surface Movement and Ground Control Systems (A-SMGCS) applications in airports including Copenhagen, Prague, Madrid, London, Paris, Atlanta and St Louis and they have also been successfully deployed as ground-based height monitoring units to support RVSM implementation by verifying the performance of barometric altimeters in dense airspace.
A recent report for Eurocontrol on Wide Area Multilateration (WAM) concludes that “Where coverage exists a WAM system will generally outperform MSSR for accuracy” and, with respect to costs found that “The hardware costs of a WAM system are (very roughly) around 50% of those of an SSR system” and “The maintenance cost of WAM systems will be much lower than MSSR as there are no rotating mechanical parts. A 6 monthly maintenance check at each site to maintain ancillary equipment such as UPS systems may be required; otherwise there is very little to do.” See, Wide Area Multilateration, Report on EATMP TRS 131/04, Eurocontrol 2005, incorporated herein by reference.
As a result, multilateration is seen as a cost-effective and high performance solution for terminal area and en-route surveillance in countries as diverse as Taiwan, Mongolia, the Czech Republic and Australia.
Furthermore, the ground stations of all commercially proven multilateration systems are also full-featured, standards-compliant ADS-B ground stations, which means that such a system is able to not only receive and decode self-reported position information, but can also triangulate on the source of the message to derive an independent position report for the same aircraft. This presents a number of opportunities in addressing ADS-B implementation challenges.
Triangulation or multilateration systems using time difference of arrival (TDOA) processing are used to track aircraft in local, regional and wide areas. These systems generally need pulse transmissions from the aircraft, which have sufficiently fast rise times in order to make a consistent time reference on the signal.
Pulse transmission systems, having sufficiently fast rise times are generally higher frequency signals, L-band or above (generally higher than 900 MHz), with sufficient bandwidth to provide the fast rise time.
Signals with sufficient frequency and bandwidth include secondary surveillance radar systems (SSR), including Mode A, Mode C, Mode S, and ADS-B.
Companies fielding triangulation systems for SSR include Sensis Corporation and ERA Systems Corporation.
While SSR signals are used for multilateration on the 1090 MHz frequency, there are others that use TDOA processing of other aircraft signals on different frequencies.
One of these is the VERA-E system manufactured by ERA Systems Corporation, assignee of the present application, and illustrated in FIGS. 1 through 4. This system is used to track aircraft over wide areas using broadband methods. Essentially the broadband aspect is achieved by using a series of antennas and receiver systems interconnected as illustrated in FIG. 4. Each sub system handles a subset of frequencies in an overall range of 1 GHz to 20 GHz. The system has the following features and capabilities:
Covertness—electronic and physical
Exploitation of electronic warfare countermeasures
Long range of detection (radio horizon is main limitation)
Tracking and Electronic Intelligence (ELINT) providing covert IFF capability
Excellent Tracking Accuracy
Coverage of both land and surface targets
Cost effective systems acquisition and life cycle cost.
FIG. 1 illustrates a deployable Broadband Receiver Unit Manufactured by ERA a.s. FIG. 2 is a close-up view of a VERA E Antenna. FIG. 3 shows a VERA E receiver unit shown on a transport for deployment. FIG. 4 illustrates VERA E architecture. Referring to FIG. 4, signals may be input from a plurality of antennas 405 comprising antennas 410, 415, 420, 430, and 425. Antenna 410 may comprise an FE SIF antenna whose input is fed to a SIF/TACAN (Selective Identification Feature/Tactical Air Navigation) receiver 445. The inputs from antennas 415, 420, 425, 430, and 435 are fed to radar band receivers 450 and 455.
The output of SIF/TACAN receiver 445 and the outputs of radar band receivers 450 and 455 are fed to a video switch and interface 460. The output of video switch and interface 460 and radar band receiver 450 is fed to the CPS system 485. Control and commands from the CPS system 490 are fed to datalink subsystem 465, which in turn comprises a plurality of data links 470, 475, and 480. The output of data link subsystem 465 in turn controls video switch and interface 460. Control and commands from CPS 490 also control radar band receivers 450 and 455 along with video switch interface 460 and SIF/TACAN receiver 445.